perf test: Add 'sqrtloop' test workload

The sqrtloop creates a child process to run an infinite loop calling
sqrt() with rand(). This is needed for ARM SPE fork test.

$ perf test -w sqrtloop

It can take an optional argument to specify how long it will run in
seconds (default: 1).

Committer notes:

Explicitely ignored the sqrt() return to fix the build on systems where
the compiler complains it isn't being used.

And added a sqrtloop specific CFLAGS to disable optimizations to make
this a bit more robust wrt dead code elimination.

Doing that a -U_FORTIFY_SOURCE needs to be added, as -O0 is incompatible
with it.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zhengjun Xing <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221116233854.1596378-8-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>